If you can't find another way, what you can do is upload the pic to Photobucket. Once it's there, you can edit it. There is a resize button there. Once you get the pic to the size you want, save it as a copy.

You can do what Kitiara said or you can use the "host image" button that is to the left of the image button. Upload an image from your computer or paste a url into the appropriate box and it will let you resize to fit the forum in the box below. Then click host it and you will get three options, choose one and allow it to copy to your clipboard, close the box and paste.